1. Field of the Invention
This invention relates to a control system for internal combustion engines, and more particularly to a control system which controls the supply of fuel injected into an intake pipe in a manner compensating for a fuel amount adhering to the inner surface of the intake pipe.
2. Prior Art
In conventional internal combustion engines of the type that fuel is injected into an intake pipe, there is a problem that some of injected fuel adheres to the inner surface of the intake pipe, so that a required amount of fuel cannot be drawn into the combustion chamber resulting in a difficulty of accurately controlling the air-fuel ratio of a mixture supplied to the engine. To solve this problem, there has been proposed a fuel supply control method which estimates a fuel amount which is to adhere to the inner surface of the intake pipe and one which is to be drawn into the combustion chamber by evaporation from the fuel adhering to the intake pipe, and determines a fuel injection amount in dependence on the estimated fuel amounts (Japanese Provisional Patent Publication (Kokai) No. 61-126337).
To improve this fuel supply control method, a supply fuel amount correction method has been proposed by the assignee of the present application, which calculates a fuel amount adhering to the inner surface of the intake pipe by the use of coefficients Ae and Be which are obtained by correcting a direct supply ratio (the ratio of a fuel amount injected into the intake pipe and directly drawn into the combustion chamber to the whole amount of fuel injected into the intake pipe) and a carry-off ratio (the ratio of a fuel amount carried off the inner surface of the intake pipe and drawn into the combustion chambers to the amount of fuel adhering to the inner surface), in dependence on the rotational speed of the engine, etc., the two ratios being determined based upon the temperature of engine coolant and pressure within the intake pipe, and corrects the fuel amount to be supplied to the combustion chamber by the use of the amount of fuel adhering to the inner surface and the coefficients Ae, Be (Japanese Patent Application No. 3-283694, and corresponding U.S. Ser. No. 07/945,489 filed Sep. 16, 1992, still pending) (Prior Art A).
Further, in internal combustion engines equipped with exhaust gas recirculation systems, it has been conventionally known to correct the supply fuel amount during operation of the exhaust gas recirculation system. Usually, an exhaust gas recirculation system has a recirculation control valve arranged across an exhaust gas recirculation passage. The control valve has a response time lag in being actuated by a control signal. In view of this response time lag, a control system of this kind has been proposed, which delays the timing of starting or terminating the correction of the supply fuel amount by a time period depending upon operating conditions of the engine, from the time the recirculation control valve is changed over from a closed position thereof to an open position or vice versa (Japanese Provisional Patent Publication (Kokai) No. 1-203641)(Prior Art B).
Also, an internal Combustion engine has been known, which is equipped with a valve timing device which changes valve operating characteristics of intake valves and exhaust valves of the engine, i.e., the opening/closing timing (valve timing) and lift thereof during operation of the engine (e.g. Japanese Patent Publication (Kokoku) No. 2-50285)(Prior Art C].
However, Prior Art A has a problem that when the engine is idling, the pressure within the intake pipe varies due to variations or fluctuations in the intake air amount, and as a result, the values of the coefficients Ae, Be for correcting the supply fuel amount vary so that the actual fuel amount supplied to the engine fluctuates, resulting in the idling condition being unstable. Particularly, in an internal combustion engine provided with an auxiliary air amount control valve for controlling the idling speed of the engine, the auxiliary air amount control valve is much inferior in dynamic characteristics such as a response time lag to fuel being supplied to the engine and accordingly, during idling, the control valve cannot quickly respond to a change in the supply fuel amount, resulting in hunting of the rotational speed of the engine and unstable engine idling.
Further, Prior Art B does not contemplate the influence of fuel adhering to the inner surface of the intake pipe upon the correction of the fuel supply amount, while Prior Art A does not contemplate the influence of the exhaust gases recirculated to the intake pipe upon the determination of the fuel injection amount. Therefore, a mere combination of the two methods cannot accurately control the air, fuel ratio of a mixture supplied to the combustion chamber to a desired value. More specifically, the control system of Prior Art B corrects the fuel supply amount merely based upon whether or not the exhaust gas recirculation is being carried out, but it does not take into consideration a delay in recirculation of exhaust gases when the recirculation control valve is changed over from a closed position thereof to an open position thereof or vice versa, that is, dynamic characteristics of the recirculation control valve and dynamic characteristics of exhaust gases being recirculated, and hence is unable to correct the fuel supply amount to a value actually required by the engine. Therefore, even by merely combining together the fuel amount correction method dependent upon the adherent fuel amount and the fuel amount correction method dependent upon the operation of the recirculation control valve, it is impossible to accurately control the amount of fuel supplied to the engine so as to attain a desired air-fuel ratio.
On the other hand, in an internal combustion engine capable of changing the operating characteristics (operating mode) of intake valves, etc., as in Prior Art C, the fuel supply amount actually required by the engine varies depending upon the valve timing etc. Therefore, to accurately control the air-fuel ratio to a desired value by correcting the fuel supply amount depending upon both the adherent fuel amount and the exhaust gas recirculation amount, the fuel correction amount has to be determined by taking into account operating modes of the intake valves and/or the exhaust valves such as valve timing thereof.